1. Field of the Invention
The present invention relates to a three-dimensional photonic structure and a method for manufacturing the three-dimensional photonic structure. In particular, the present invention relates to a method for manufacturing a three-dimensional photonic structure having a plurality of inorganic members disposed at specific locations in a resin and to a three-dimensional photonic structure produced by the method.
2. Description of the Related Art
Photonic crystals include material bodies periodically disposed in a specific substance, each of the material bodies having a dielectric constant that is different from the dielectric constant of the specific substance, and the photonic crystals completely reflect electromagnetic waves having specific wavelengths due to the mutual interference of electromagnetic waves. The frequencies of such electromagnetic waves that are completely reflected are in a specific range, which is called a “photonic band gap”.
When an electromagnetic wave enters a periodic dielectric structure, two kinds of standing waves are produced by Bragg diffraction. One standing wave oscillates in a region having a low dielectric constant, and another standing wave oscillates in a region having a high dielectric constant. The former has an energy level that is greater than that of the latter. That is, waves having energy levels between energy levels of the two standing waves, which have different modes from each other, cannot enter the crystal, and therefore, a photonic band gap is produced.
Since photonic band gaps, as described above, are produced by Bragg diffraction, it is necessary that lattice constants, which are repetition periods in periodic structures, correspond to wavelengths. An increase in the difference between dielectric constants increases the difference between vibrational energy levels in dielectric phases, thus increasing the photonic band gap. A higher dielectric constant reduces vibrational energy. As a result, the photonic band gap shifts to lower frequencies.
Various photonic crystals have been developed. To completely reflect a three-dimensional electromagnetic wave, it is necessary to produce a photonic band gap in all directions. A photonic crystal that meets such a demand includes, for example, a diamond structure. However, since diamond structures are complicated, it is difficult to manufacture such a diamond structure. A process for manufacturing a photonic crystal by stereolithography is presently being examined.
Examples of processes for manufacturing photonic crystals by stereolithography include the following approaches.
First, for example, Japanese Unexamined Patent Application Publication No. 2000-341031 discloses a process for manufacturing a photonic crystal as follows: two-dimensional basic structures each having a plurality of rods are formed and successively stacked to produce a photonic crystal by stereolithography with a composite material composed of a photocurable resin containing a powdered dielectric ceramic.
Second, for example, Japanese Translation Patent Publication No. 2001-502256 discloses a process in which a three-dimensional component, which is composed of a photocurable resin, having voids formed at predetermined locations is manufactured and then a composite material composed of a resin into which dielectric ceramic powders are dispersed is charged into the voids.
For example, Japanese Unexamined Patent Application Publication No. 2001-237616 discloses a process, where stereolithography is not applied, in which a coating containing a powdered low-dielectric ceramic is printed in a dot pattern on a green sheet containing a powdered high-dielectric ceramic and then the resulting green sheets are stacked and sintered.
However, the above-described processes have problems.
It is difficult to manufacture a photonic crystal that contains a low-loss dielectric having high-dielectric constant by these processes disclosed in Japanese Unexamined Patent Application Publication No. 2000-341031 and Japanese Translation Patent Publication No. 2001-502256 because, in the processes disclosed in these Patent Publications, a composite material composed of a resin into which a powdered dielectric ceramic is dispersed is used as a dielectric.
A process disclosed in Japanese Unexamined Patent Application Publication No. 2000-341031 applies the difference between the dielectric constant of a composite material composed of a resin mixed with a powdered dielectric ceramic and the dielectric constant of air that is present between rods composed of the composite material. In this case, since the dielectric constant of the composite material is determined by the mixing ratio of the resin and the powdered dielectric ceramic, the above-described difference between these dielectric constants is only determined by the dielectric constant of the composite material. As a result, the range of the resulting photonic band gap is limited.
In each process disclosed in Japanese Unexamined Patent Application Publication No. 2000-341031 and Japanese Translation Patent Publication No. 2001-502256, it is necessary to supply a liquid photocurable resin so as to form a layer having a predetermined thickness on a platform by gradually lowering the platform. Accordingly, the use of a liquid photocurable resin having a high viscosity barely forms any shape. Hence, in a process, particularly disclosed in Japanese Translation Patent Publication No. 2001-502256, when a powdered dielectric ceramic is mixed with a liquid photocurable resin, the content of the powdered dielectric ceramic is limited, i.e., to about 60% at an upper limit. Even when the content of the powdered dielectric ceramic is about 60%, the dielectric constant of the composite material is about ¼ or less of that of the dielectric ceramic used. Therefore, high contrast photonic crystals cannot be satisfactorily produced.
On the other hand, in a process disclosed in Japanese Unexamined Patent Application Publication No. 2001-237616, since dots composed of a powdered low-dielectric ceramic are merely printed, these dots cannot be formed in substantially three-dimensional shapes. Furthermore, these dots cannot be disposed at desired locations along the stacking direction because of the limitation caused by the thickness of the green sheet. In addition, since the green sheets and the dots shrink when sintering, it is difficult dispose the dots at a desired period in the sintered body and so as to form a desired photonic band gap.